docs(pi): name the loader mechanism behind the default-export requirement

Two contributors independently sent the same fix for #1550 while ours was
already in CI (#1553 by @musichen, who also reported the bug, and #1564 by
@henri-edh). Rather than close both as duplicates, this takes the part of
theirs that ours was missing.

@henri-edh's comment names the actual mechanism: Pi loads an extension via
jiti.import(path, { default: true }) and rejects anything that is not a
function, so a named-only export hands the loader the module namespace object
and the install fails with "Extension does not export a valid factory
function". Ours said "Pi calls the default export" — true, but it does not tell
the next reader why a named export cannot simply be added alongside, which is
exactly the change someone would try. Their wording is now ours.

The assertion is tightened the same way, from `export default function` to
`export default function (pi)`, so a default export with the wrong shape cannot
satisfy it.

Co-Authored-By: musichen <musichen@users.noreply.github.com>
Co-Authored-By: henri-edh <henri-edh@users.noreply.github.com>
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

src/cli/client_adapter.c

@@ -157,12 +157,13 @@ char *cbm_client_adapter_pi(const char *binary_path) {
     /* Register every tool the registry advertises — never a hand-picked subset,
      * which is the drift this generator exists to prevent.
      *
-     * DEFAULT export, and it must stay that way: Pi loads an extension by
-     * calling its default export as a factory. A named `register` export made
-     * the file fail to load, and a Pi extension that fails to load takes EVERY
-     * pi command down with it — `pi doctor` included — not just cbm's tools
-     * (#1550). The named binding is kept alongside for anything that imported
-     * it by name. */
+     * DEFAULT export, and it must stay that way. Pi loads an extension via
+     * jiti.import(path, { default: true }) and rejects anything that is not a
+     * function, so a named `export function register(pi)` hands the loader the
+     * module namespace object instead and every install fails with "Extension
+     * does not export a valid factory function". A Pi extension that fails to
+     * load takes EVERY pi command down with it — `pi doctor` included — not
+     * just cbm's tools (#1550). */
     sb_append(&sb, "export default function (pi) {\n");
     for (int i = 0; i < count; i++) {
         const char *name = cbm_mcp_tool_name(i);

tests/test_agent_clients.c

@@ -1106,7 +1106,7 @@ TEST(client_adapter_pi_registers_every_registry_tool) {
 TEST(client_adapter_pi_default_exports_its_factory_issue1550) {
     char *js = cbm_client_adapter_pi("/usr/local/bin/codebase-memory-mcp");
     ASSERT_NOT_NULL(js);
-    ASSERT_NOT_NULL(strstr(js, "export default function"));
+    ASSERT_NOT_NULL(strstr(js, "export default function (pi)"));
     /* The old shape must be gone: a bare `export function register(pi)` is the
      * exact form Pi rejects. */
     ASSERT_NULL(strstr(js, "export function register(pi)"));